Airlines continually tweak the order in which they board passengers. They’ve tried everything from a free-for-all to window-seats-and-back-of-plane first, to random boarding. One airline has even tried boarding passengers without carry-on bags first, which makes sense since pausing to find overhead bin space jams up the boarding of everyone behind. JetBlue recently changed its boarding procedure, so it’s a good time to take a look at how the different U.S.-based airlines are getting passengers from gates to seats.

Why is this an issue? Overhead bin space. If you’re the last to board, then the chances increase that cabin crew will make you gate-check your carry-on bag as the bins get filled by those who boarded ahead of you.

An American Eagle Embraer E170 jet passes in front of the iconic Hollywood sign while on final approach to Los Angeles International Airport on Nov. 8, 2015. Jeremy Dwyer-Lindgren, Special for USA TODAY

The skyline of New York. American counts its operations operations at JFK and LaGuardia airports as a combined hub, though the airports are separated by a considerable cab ride during normal traffic conditions.LaGuardia was American's eighth-busiest hub (by passengers) in 2015. JFK was ninth. Mark Lennihan, AP

Third group: Active military and passengers with children in car seats or strollers

Fourth group: JetBlue calls the next group “C”. Although logic would suggest that these passengers are seated at the back of the plane in window seats, a jetBlue spokesperson says, “The exact group is determined upon check-in and takes a number of factors into consideration.” Anecdotally, passengers have said that window seats are boarded first.

Fifth group (Group D): JetBlue doesn’t specify exactly which passengers are assigned to this group

Sixth group (Group E): JetBlue doesn’t specify exactly which passengers are assigned to this group

Southwest

Pre-boarding: Customers who have a specific seating need to accommodate their disability or need assistance in boarding the aircraft or stowing an assistive device. Customers who are traveling with assistance and emotional support animals qualify.

First group: Passengers with and “A” group boarding pass and customers on “business select” fares

Chicago's skyline is seen as a United Airlines jet takes off from Chicago O'Hare on Sept. 19, 2014. In the distance is the Willis (Sears) Tower that's home to United's official headquarters. Scott Olson, Getty Images